It is well known that communications circuits carrying voice conversations are idle over significant periods of time. On a typical telephone circuit, each call in progress occupies a full-duplex trunk circuit. Each such circuit is dedicated to the conversation during the entire duration of the call, yet typically each party is active (i.e., speaking) only about half of the time. Thus, on the average at least half of the transmission capacity in a telephone circuit typically is idle. Furthermore, when such communications circuits form part of a telephone switching network, additional periods exist in which the communications circuits are unassigned to voice conversations and are idle. The cumulative effect of these factors is that 65 percent or more of the transmission capacity in voice communications networks is wasted.
The prior art contains examples of systems which exploit this phenomenon to allow sharing of a communications circuit between voice conversations and non-real-time data transmission. In general, these systems comprise a means for monitoring, by the transmitting apparatus, voice activity on the communications circuit and for detecting when voice activity drops below a preset threshold; the transmitting apparatus further switching between the transmission of voice information and data information; and sending a signal to the receiving apparatus to commence receiving data transmissions. Such devices send a signal from the transmitting apparatus to the receiving apparatus to resume receiving voice transmissions when voice activity resumes at the transmitting apparatus; and the switching between the receiving of voice information and data information is reinstated by the receiving apparatus in response to this signal. A key problem to be solved by any such system is in signalling between the transmitting apparatus and receiving apparatus to allow the proper separation of voice information and data information by the receiving apparatus. In a practical system, the signalling method must be immune to false signalling due to noise on the transmission line; it must not add unduly to the overhead (i.e., non-end user information) that must be transmitted; and must not impose restrictions on the allowable information content of either the voice information or data information to be carried on the shared circuit.
One early method reserved one bit in each PCM code word for signalling. A "zero" in this bit position indicated one type of information, for example voice information, was contained in the remaining bit positions of the code word, while a "one" indicated the other information type. This method, while effective, reduces the number of voice quantizing levels which can be represented by a voice codeword of given length, resulting in a reduction in quality of the circuit for carrying voice conversations.
Other systems in the prior art have improved on this by inserting special signalling characters onto the communications circuit to indicate changes from voice to data transmission of the transmitting apparatus, and vice versa. While such a system markedly improves the quality of the circuit by allowing all bits to be used for carrying quantization level information, it has the drawback that the signalling characters cannot be contained in the voice and data information since this would falsely signal the receiving apparatus to change from voice to data reception, or vice versa. Such an occurrence would cause the receiving apparatus erroneously to interpret data information as voice information and vice versa, thereby intermixing the two separate forms of information.
This problem has been solved in the past by prohibiting the signalling characters from appearing in the digitalized voice information or in the data information entered into the system by the end user. From the viewpoint of the end user, the communications circuit is no longer transparent for a selected application, because the user is prevented from placing the signalling characters in the voice information or data information to be carried by the circuit. This method was acceptable in older data systems, intended primarily for the transmission of teletype and telegraph signals based on pre-defined and well-structured coding schemes, which inherently contained a number of reserved signalling characters. However, it is not practical for use with more modern computer and data systems, which generally will transmit arbitrary, unrestricted data information occasionally containing the signalling characters. For voice, this method removes at least one quantization level from use for digitizing voice, resulting in a slight reduction in quality of voice conversations carried on the circuit.
An additional difficulty with such a method is that it is susceptible to false signalling caused by noise in the communications circuit. Errors thus induced can with some probability corrupt a genuine signalling character, destroying its appearance as a signalling character and causing it to be interpreted by the receiving apparatus as either voice information or data information. This causes the transmitting apparatus and receiving apparatus to enter different states, so that voice information is erroneously interpreted to be data information by the receiving apparatus, and vice versa. Similarly, voice information or data information may be corrupted by errors on the communications circuit in such a way that a signalling character is falsely created when none was sent by the transmitting apparatus, again causing the transmitting apparatus and receiving apparatus to enter different states.